1. Field of the Invention
The present invention relates to the generation of coherent light and particularly to improvements in gas lasers. More specifically, the present invention is directed to enhancing the efficiency of pulsed discharge gas lasers, and especially transversely excited lasers, through co-ionization employing low and high energy sources. Accordingly, the objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited to use in and as a transversely excited laser. Transversely excited (TE) gas lasers which operate in a pulsed or CW (continuous wave) mode are known in the art. It is known in the art that, before such TE lasers can be efficiently energized with a dc pulse, preionization must be supplied to the region which subsequently becomes the laser gain medium. It is also known that, subsequent to excitation, gas recirculation or replacement of the laser medium must be provided beyond a repetition frequency of about a few pulses per second to prevent discharge arcing.
FIG. 1 schematically illustrates the major elements of a conventional ultra-violet preionized, thyratron switched, transversely excited CO.sub.2 laser. These elements include a pair of solid metal main discharge electrodes 1 and 2, at least one of the electrodes 2 being shaped in the interest of establishment of a uniform electric field in the region between the electrode pair. Additionally, the prior art TE laser devices have customarily employed a series of preionizing electrodes 4 located adjacent to, and typically downstream of in the direction of gas flow, the space between the electrodes where the discharge will occur. As will be briefly discussed below, preionization can be accomplished in several different ways. Thus, by way of example, the preionizer electrodes may be arranged as a series of oppositely positioned "pins". Typically, the sets of pins are mounted from the upper and lower interior surfaces of the laser vacuum envelope. The application of a high voltage pulse between the pins of the sets causes the generation of an intense linear array of sparks. The ultra-violet light from these sparks, in turn, generates photo-electrons in the volume adjacent to the spark array. The energy disposition in the sparks is typically about 10% of the total energy subsequently applied to the main discharge electrodes to generate laser oscillation.
Continuing to discuss the prior art as schematically represented in FIG. 1, the energy for generating the linear array of sparks is typically stored in a capacitance, such as capacitor C.sub.s ', the capacitance being charged from high voltage source V.sub.s ' to the appropriate voltage through an inductance such as indicated at L'. The inductance L' is chosen, taking the pulse repetition frequency into account, such that its reactance will not prevent the capacitance C.sub.s ' from charging in the laser inter-pulse period, while at the same time the reactance will be sufficiently high as to not shunt current away from the spark array. The voltage pulses delivered to the spark array may, for example, be provided by exercising control over a switching device SW1 such as a thyratron. Thus, in the prior art system depicted, the application of a positive pulse to the control grid of the thyratron results in a negative going pulse being generated and applied to the spark array, i.e., the anode voltage of the thyratron is driven to an impedance level of a few ohms above ground for a period measured in tens or hundreds of nanoseconds. When the optimum ionization level is developed in the region between the two main discharge electrodes, due to the absorption of the uv spark protons and subsequent generation of photo-electrons, a second voltage pulse from a main storage capacitance C.sub.s is applied across the laser main discharge electrodes. The energy delivered to the discharge volume from capacitance C.sub.s is usually on the order of ten times that of the preionization energy. The control of the coupling of energy into the laser gain volume from capacitance C.sub.s is achieved by the use of a second switch (thyratron) SW2.
While various modifications and improvements to the circuit depicted schematically in FIG. 1 have been proposed and implemented, the net result of such improvements has been to only improve the laser output from a given device volume at the expense of more complicated electrode structure and discharge circuit complexity. Thus, all of the prior art preionized pulsed TE lasers share several fundamental characteristics. Firstly, the magnitude of the voltage pulse which generates the initial preionization sparks must be at a level which is substantially above the breakdown level of the laser gas mix taking into account the pin shape and pin spacing. Further, because the level of photoelectrons available for preionization is not volumetrically dense, as a result of the fact that a small fraction of the preionizer circuit energy ends up as uv photons, the voltage pulse coming from the energy stored in the main capacitance C.sub.s (or the voltage to which the capacitance C.sub.s must be charged) must also be above the gas breakdown level taking the main discharge electrode spacing into account. If the C.sub.s capacitance voltage is not above the breakdown level, all of the energy stored in this capacitance will be dumped either into the charging inductor L or into the switching device SW2 when the switching device in the main discharge circuit is activated. Further, experience has shown that, beyond a discharge repetition frequency of about one or two pulses per second, prior art pulsed TE gas lasers require some sort of intravacuum gas recirculation or convective gas flow to prevent discharge arcing on the second consecutive discharge pulse. It is to be noted that this requirement for gas recirculation cannot be avoided by achieving preionization through the use of corona discharge or by generating photo-electrons through the use of an x-ray source or flash lamps having an output rich in uv emission.
To summarize the above, the prior art uv or corona preionization techniques all require more than two electrodes in the laser vacuum envelope and, typically, require such a substantial number of electrodes as to introduce significant complications and compromises in material selection, material compatibility and overall device design. Further, the prior art preionization schemes all require intravacuum gas flow in order to reliably achieve pulse repetition frequencies of greater than one or two pulses per second and, accordingly, require comparatively large device volume and have both a life expectancy and reliability which is suspect due to the necessity of having rotating mechanical elements interior to the vacuum envelope. Additionally, the prior preionization schemes have all required the use of at least one active switching device in the main discharge circuit and thereby have been characterized by circuit complexity and limited service life.
To further discuss the prior art, a "pulser"/"sustainer" system for CO.sub.2 transverse laser excitation is discussed in an article by J. P. Reilly which appeared in the Journal of Applied Physics, Volume 43, No. 8, August 1972. The technique discussed in the Reilly article employs a low energy level pulsed discharge to provide ionization to initiate the deposition of energy from an "unswitched" more energetic discharge circuit. The approach discussed in this article employed a combination of a highly overvoltaged "pulser" or preionizer discharge pulse, numerous interlevered pulser/sustainer electrodes and mach 0.2 intravacuum gas flow. The principles of operation as described in the Reilly article are very similar to electron beam controlled laser devices subsequently described by Fenstermacher et al (see Applied Physics Letters, Volume 20, pages 56-60, 1972) and Stratton et al (Journal of Quantum Electronics, Volume QE 9, No. 1, 1973). The pulser portion of an electron beam controlled laser device comprises a hot cathode electron gun located in a "hard" vacuum chamber. One wall of this chamber is at least in part defined by a titanium foil supported by a metal grid, the foil being substantially transparent to accelerated electrons from the electron gun. In actual practice, electron beam type ionizer sustainer devices have suspect reliability in view of the very high voltages required to accelerate the electron beam, the types of switches needed to activate the electron gun and the fragile nature of the foil electrode. Additionally, the location of the electron gun vacuum chamber in one of the walls of the laser vacuum envelope severely impairs heat removal from the lasing gas. Thus, it is believed that, as is the case with the above-discussed uv or corona preionization schemes, intravacuum gas flow is required in order to achieve pulse repetition frequencies beyond about one or two pulses per second.
In summary, the pulsed E-beam, UV, flashlamp or corona preionized TE lasers apply one or more types of preionization and main discharge excitation by multiplexing several such sources in time (one source activated after another) or space (different electrodes, different regions of gas).
In similar fashion to the pulsed transverse gas lasers, transverse cw dc excitation between only two simple extended metal electrodes, in a sealed-off non-recirculating device, has proven to be impractical. Without some turbulence due to gas flow or some other gas discharge destabilizing influence, for example due to charged particle-magnetic field induced forces, a desirable glow discharge in the laser medium rapidly localizes to one small region between the electrodes and collapses into an undesirable arc. The beneficial features of simplicity and reduced laser cost attendant to using only two electrodes in a transversely excited cw gas laser are so compelling that, over the last decade, much work has gone into teaching how to generate such two electrode cw discharges. In order to prevent the collapse of the desirable glow discharge into an arc, some mechanism to prevent the formation of the arc is necessary. One such mechanism might be the use of intravacuum convective gas flow which will transport the high temperature gas in the early stages of the glow to arc transition to an adjacent region of the gas thereby inhibiting the development of an arc. Another mechanism would be to use an externally applied magnetic field to induce forces on the charged particles in the discharge in a manner that destabilizes the arc. All of these techniques run counter to the desire to provide a less complicated device. The the past decade the thrust of the sealed-off laser development has been to provide a mechanism which prevents the formation of the glow to arc transition at a more fundamental level. Thus, the teachings in U.S. Pat. No. 4,169,251 by Laakmann point out the need to use an "excitation frequency [col. 2 line 51] sufficiently high to ensure negligible interaction of discharge electrons with the electric field-applying electrodes". Or, as indicated in U.S. Pat. No. 4,373,202 by K. D. Laakmann and P. Laakmann, [col. 2, line 38]"the overall laser efficiency for the transverse RF discharge of the prior art device suffers due to low laser head efficiency if the RF drive frequency is below the desirable minimum", thereby necessitating the use of the longitudinal RF discharge excitation approach as taught in that patent. As indicated in the invited paper entitled "RF Excited Waveguide CO.sub.2 Laser Technology" at the Lasers '82 Conference by Chenausky and Newman, "The principal distinguishing feature of transverse RF excitation is that the `transient basis` of the applied discharge voltage is supplied in the form of a polarity reversing AC electric field with a frequency typically in the range of 20 MHz. to 200 MHz. The only general requirement which seems to be relevant is that for any particular discharge geometry and operating conditions, the field reversal time must be short relative to the growth time of an unwanted plasma instability." The advantages of transverse cw RF excitation are amply discussed in U.S. Pat. Nos. 4,363,126 [Chenausky et al], 4,438,514 [Chenausky et al], 4,443,877 [Chenausky and Newman] and 4,719,640 [Chenausky et al].
Therefore, based on these teachings, substantial transverse dc excitation between two extended metal electrodes will be unsuccessful because either the frequency will be too low (DC), resulting in a non-negligible interaction with the discharge electrodes or will lack some destabilizing mechanism for the formation of an arc, such as the transient nature of the polarity reversing discharge electric field. Transient transverse dc excitation can be successful only if it is preceded with at least space multiplexed and time multiplexed preionization means.